IS-IS multitopology support

IS-IS multitopology support for IPv6 is a feature that allows the protocol to operate using both IPv4 and IPv6 simultaneously. It removes some restrictions that did not allow this using conventional IS-IS. Specifically, Cisco describes it in its documentation like so:

IS-IS multitopology support for IPv6 allows IS-IS to maintain a set of independent topologies within a single area or domain. This mode removes the restriction that all interfaces on which IS-IS is configured must support the identical set of network address families. It also removes the restriction that all routers in the IS-IS area (for Level 1 routing) or domain (for Level 2 routing) must support the identical set of network layer address families. Because multiple SPFs are performed, one for each configured topology, it is sufficient that connectivity exists among a subset of the routers in the area or domain for a given network address family to be routable.